Stepping motors generally have either magnetically permeable rotors or permanent magnet rotors. Coils are generally mounted with their axes radially oriented around the rotor and fixed to the stator of the stepping motor. Customarily, several coils spaced about the stator are interconnected and energized simultaneously in order to position the rotor in a predetermined angular relationship with respect to these coils.
In a variable-reluctance-rotor stepping motor, torque is developed by the rotor until the rotor is positioned so as to minimize the air gap between the rotor and the stator core adjacent the energized coils. In a permanent magnet motor, the rotor moves so as to place a permanent magnet pole as close as possible to the opposite pole generated electromagnetically by an appropriate coil. Many sets of these coils can be positioned around the stator with each set energized in succession in order to advance the stepping motor by small angular increments or steps as one set of coils is de-energized and another set of coils is energized. In systems in which a stepping motor is driving a load such as the printing device disclosed in U.S. Pat. No. 3,982,622 granted on Sept. 28, 1976, to J. A. Bellino et al., that can have both inertial and friction loading, the stepping speed of the rotor resulting from energizing one set of coils after another is usually limited to a worse-case situation in order to prevent advancing the coil energization state so fast as to outpace the rotor of the stepping motor and thus cause the rotor to lose synchronism with the coils.
Due to the highly inductive nature of the stepping coils, motor torque control is difficult since a lower torque generally requires either lowering the voltage supplied to the stepping motor and its driver amplifiers or increasing resistance placed in series with the coils in order to limit the peak current through the coil. A high supply voltage is desirable for high energy concentration at inertial start-up of the motor yet overcurrent is not desired. In addition, the insertion of electrical resistance also shows inductive response by effectively dropping the voltage across the coil as coil current builds up.
Therefore, it is an object of the present invention to control the speed of a stepping motor.
It is another object of the present invention to control the power supplied to a stepping motor.